The Third Generation Partnership Project (3GPP) unites six telecommunications standards bodies, known as “Organizational Partners,” and provides their members with a stable environment to produce the highly successful Reports and Specifications that define 3GPP technologies. A mobile device, also called a User Equipment (UE), may operate in a wireless communication network that provides high-speed data and/or voice communications. The wireless communication networks may implement circuit-switched (CS) and/or packet-switched (PS) communication protocols to provide various services. For example, the UE may operate in accordance with one or more radio technology such as Universal Terrestrial Radio Access (UTRA), Evolved UTRA (E-UTRA), and Global System for Mobile Communications (GSM) as part of an Universal Mobile Telecommunication System (UMTS). UTRA, E-UTRA, GSM, UMTS and LTE (Long Term Evolution) are described in specification documents from 3GPP. These various radio technologies and standards are known in the art.
The Evolved Packet Core (EPC) is the latest evolution of the 3GPP core network architecture first introduced in Release 8 of the standard. In EPC, the user data and the signaling data are separated into the user plane and the control plane. The EPC is composed of four basic network elements: the Serving Gateway (SGW), the Packet Data Network Gateway (PDN GW or PGW), the Mobility Management Entity (MME), and the Home Subscriber Server (HSS). The EPC is connected to external networks, which can include the IP Multimedia Core Network Subsystem (IMS).
The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem (IMS) is an architectural framework for delivering IP multimedia services. Historically, mobile phones have provided voice call services over a switched-circuit-style network, rather than strictly over an IP packet-switched network. Alternative methods of delivering voice or other multimedia services over IP have become available on smartphones (e.g., VoIP), but they have not become standardized across the industry. IMS is an architectural framework to provide standardization. Session Initiation Protocol or SIP is the primary protocol used in IMS.
Long Term Evolution (LTE) is a high-speed high-capacity IP-based data standard for mobile devices. Because LTE systems do not support voice calls, Circuit Switched Fallback (CSFB) specified by 3GPP standards TS 23.272 is executed to handle these voice calls. When a mobile device camped on LTE radio receives a voice call page request over the LTE system, the mobile device must fall back to the CS radio to complete the call.